Field of the Disclosure
This disclosed system relates generally to an antenna structure including an antenna and a frequency selective impedance surface surrounding the antenna printed on a flexible substrate and, more particularly, to a dual-band co-planar (CPW) antenna structure mounted to vehicle glass and including a frequency selective impedance surface surrounding an antenna that reduces the effects of surface waves.
Discussion of the Related Art
Modern vehicles employ various and many types of antennas to receive and transmit signals for different communications systems, such as terrestrial radio (AM/FM), cellular telephone, satellite radio, dedicated short range communications (DSRC), WiFi, GPS, etc. Further, cellular telephone is expanding into 4G long term evolution (LTE) that requires two antennas to provide multiple-input multiple-output (MIMO) operation. The antennas used for these systems are often mounted to a roof of the vehicle so as to provide maximum reception capability. Further, many of these antennas are often integrated into a common structure and housing mounted to the roof of the vehicle, such as a “shark-fin” roof mounted antenna module. As the number of antennas on a vehicle increases, the size of the structures required to house all of the antennas in an efficient manner and provide maximum reception capability also increases, which interferes with the design and styling of the vehicle. Because of this, automotive engineers and designers are looking for other suitable areas on the vehicle to place antennas that may not interfere with vehicle design and structure.
One of those areas is vehicle glass, such as the vehicle windshield, which has benefits because glass makes a good dielectric substrate for an antenna. For example, it is known in the art to print AM and FM antennas on the glass of a vehicle, where the printed antennas are fabricated with the glass as a single piece. However, these known antennas are generally limited in that they could only be placed in a vehicle windshield or other glass surface in areas where viewing through the glass was not necessary.
When an antenna is placed on a dielectric substrate energy generated by the antenna for both transmission and reception purposes gets coupled at least in part into the substrate where surface waves can be created. For example, the thickness of an automotive windshield and other glass is typically in the range of 3-5 mm, which is electrically thick at the 5.8 GHz WiFi frequency band. When antennas are flush mounted to electrically thick substrates surface waves arise that can result in undesired scattering and a reduction in antenna efficiency and gain. Those surface waves expand out from the antenna along the substrate until they reach the edge of the substrate, where they are either radiated in an undesirable fashion or dissipated or coupled into conductive structures, such as where vehicle glass is coupled to the metallic vehicle body. Thus, much of the energy that is to be radiated by the antenna is lost, reducing the efficiency and performance of the antenna.
Surface waves occur in situations where an electrically thick substrate compared to the signal wavelength supports surfaces waves. Surface waves can be created by printed antennas or antennas that are flush mounted to a substrate. This can be particularly problematic for wideband antennas, where the substrate happens to be electrically thick at some frequencies and electrically thin at other frequencies within the operating bandwidth of the antenna. Surface waves can also be created by incident energy from a distant source, that is, sources not directly mounted on the structure of interest. The presence of surface waves can result in undesired scattering, reduction in antenna gain, and can damage or interfere with the operation of other sensitive electronics on the same structure.
Holographic and sinusoidally modulated impedance surfaces have been used to control surface waves. A bound surface wave mode is perturbed in a sinusoidal fashion to create slow leakage and directive radiation. To date, these surfaces have not been used as an integrated or retrofitted treatment to a separate antenna. Typically, holographic and sinusoidally modulated surfaces are antennas that must be customized based on their excitation source to achieve the specified radiation angle, and are designed to control the transverse magnetic (TM) mode and required grounded substrates for this reason. Versions of the holographic antenna that do not require a grounded substrate and control the transverse electric (TE) mode have been demonstrated, but they required the thickness of the substrate to be varied in order to achieve radiation.